To Kill a Mockingbird plays of father/attorney Atticus Finch's admonition to kill no mockingbirds. In various parts of the book, Atricus's complete line is that it is a sin to kill a mockingbird because a mockingbird causes us no trouble yet provides us a song.
Tom Robinson is a black man who works hard and offers help when he can. This mockingbird is needlessly killed.
Boo Radley is a misunderstood eccentric who kills Bob Ewell while defending Jem and Scout. When the sheriff decides to ignore certain parts of what happened, the mockingbird in the person of Boo Radley is spared.